Add DBus activation as a search provider

Apparently in some cases GNOME Shell will traceback if a search provider
is not activatable. Provide a DBus service file for ourselves and a
--no-default-window command line option to support that. Activating
when we're not running may eventually be useful for searching across
history.

Consistently use org.gnome.hotssh as a namespace, not org.gnome.HotSsh.

https://bugzilla.gnome.org/show_bug.cgi?id=712345